1. Field of the Invention
The present invention relates to a liquid level measuring apparatus and in particular, to a liquid level measuring apparatus for measuring liquid level displacements by using fine oscillations.
2. Description of the Related Art
Various liquid level measuring apparatuses have been suggested. For example, Japanese Utility Model 6-69276 discloses a liquid level measuring apparatus for optically measuring a liquid level. Moreover, Japanese Patent Publication 11-30544 discloses a capacitance-type liquid level measuring apparatus capable of measuring a liquid level in a tank continuously and in real time. Furthermore, Japanese Patent Publication 11-153471 discloses a liquid information measuring apparatus.
Utilizing a single acoustic wave oscillator to measure a liquid level as well as an acoustic wave propagation time.
Moreover, Japanese Patent Publication 63-67545 discloses a specific gravity measuring apparatus shown in FIG. 7. Referring to FIG. 7, in this specific gravity measuring apparatus, when the specific gravity of liquid 103 is changed, the buoyancy functioning on a floating member 102 and a reference member 111 is changed. This displaces the floating member 102 in relation to the reference member 111. That is a specific gravity change is converted into a vertical displacement of the floating member 102. This displacement is detected by a differential transformer 114 and an electric signal corresponding to the displacement is output. This electric signal is used to determine the specific gravity of the liquid 103. This specific gravity measuring apparatus can be applied to an oscillating liquid level measuring apparatus.
Since the liquid level measuring apparatus using the floating member and the differential transformer is costly, there has been developed a liquid level measuring apparatus using a contact-type variable resistor instead of the differential transformer.
However, the contact-type variable resistor has a problem in that the contact surface is worn away by repeated slide motion and the variable resistor cannot maintain linearity. In general, it is said to have a service life of 100,000 to 1,000,000 cycles. However, there is a problem in that when an oscillating liquid level is measured continuously, the variable resistor repeats sliding motion locally and that portion alone experiences extreme local wearing.
The conventional liquid level measuring apparatus uses a number of parts, each of which is expensive, increasing the total cost.
Moreover, in the case of the liquid level measuring apparatus using the contact-type variable resistor, its service life is shortened when the liquid level to be measured is oscillating. This is because while the liquid level is oscillating, the variable resistor repeats sliding motion at a particular location, which is easily worn away.
It is therefore an object of the present invention to provide an oscillating liquid level measuring apparatus having a long service life at a reasonable cost.
The oscillating liquid level measuring apparatus according to the present invention comprises a liquid level measuring unit for measuring a displacement of a floating member floating on a liquid surface while ignoring displacement changes not greater than a predetermined amplitude.
Moreover, the liquid level measuring unit includes: a floating member having a specific gravity smaller than that of the liquid and floating on a surface of liquid in a tank; a displacement amount measuring unit arranged on the top of the tank for measuring a perpendicular displacement of the floating member; a transmitting member extending from the floating member through the top of the tank for transmitting the perpendicular displacement of the floating member to the displacement amount measuring unit; and holders arranged to sandwich the transmitting members for restricting the movement of the floating member only in the perpendicular direction, wherein a connection portion between the transmitting member and the displacement amount measuring unit has a predetermined allowance in the displacement direction of the floating member.
Furthermore, the predetermined allowance may be equal to the predetermined amplitude to be ignored.
Furthermore, the displacement amount measuring unit may be composed of a variable resistor.
Furthermore, the displacement amount measuring unit may be composed of an encoder.
Another aspect of the present invention provides a liquid developing apparatus using the liquid level measuring apparatus according to the present invention.